1. Field of the Invention
The present invention relates to an optical module employed for a high-density wavelength division multiplex optical transmission system.
2. Description of the Prior Art
FIG. 17 is a schematic sectional view showing the structure of a conventional optical module. Referring to FIG. 17, the conventional optical module mainly includes a semiconductor laser 20 converting an electric signal to an optical signal, a thermistor 70 serving as temperature detection means, a mounting unit 50 mounting the semiconductor laser 20 and the thermistor 70, a thermoelectric cooling element 80 for heating/cooling the mounting unit 50 for controlling the temperature thereof, a driver IC (integrated circuit) 30, a feeder line 110, a package 60 storing these members, and a signal input connector 40 electrically connected to an electric signal input/output unit of the package 60.
The feeder line 110 is employed for electrically connecting the semiconductor laser 20 and the electric signal input/output unit of the package 60 with each other. The driver IC 30 is electrically connected between the feeder line 110 and the electric signal input/output unit of the package 60 for amplifying the electric signal input in the semiconductor laser 20.
FIGS. 18 and 19 are a perspective view and a sectional view schematically showing the structure of the feeder line 110 respectively. Referring to FIGS. 18 and 19, the feeder line 110 is a microstrip line prepared by forming conductor films 102 and 103 on an alumina ceramic substrate 101 mainly composed of aluminum oxide. Each of the conductor films 102 and 103 consists of a multilayer structure including at least two layers, i.e., a lower conductor layer 102b or 103b actually fed with a high-frequency electric signal and a gold plating layer 102a or 103a necessary for soldering or wire bonding.
A wavelength division multiplex transmission system is watched with interest as an application of such an optical module. This system, multiplexing a plurality of signals in an optical wavelength region and transmitting the same, readily increases the capacity of an optical communication system. Recently, a high-density wavelength division multiplex transmission system narrowing the multiplexed wavelength interval to 200 GHz or 100 GHZ has been defined under international standards for attaining a higher capacity. The optical module must have a sufficiently stable wavelength (preferably not more than about 1/100 the wavelength interval) with respect to this wavelength interval.
In the optical module shown in FIG. 17, heat flows into the mounting unit 50 mounting the semiconductor laser 20 and the thermistor 70 mainly through the aforementioned feeder line 110. The temperature of the optical module is so controlled that the temperature detected by the thermistor 70 is constant.
In practice, however, the portion provided with the semiconductor laser 20 and the portion provided with the thermistor 70 are different in thermal resistance from each other as viewed from the heat inflow path, to exhibit different temperatures, as shown in FIG. 20. When the ambient temperature for the optical module changes, therefore, the temperature of the semiconductor laser 20 disadvantageously changes even if the temperature of the optical module is so controlled that the temperature detected by the thermistor 70 is regularly constant.
Referring to FIG. 20, the descending solid line shows temperature distribution in the case where the package 60 has a higher temperature than the semiconductor laser 20 while the ascending solid line shows temperature distribution in the case where the package 60 has a lower temperature than the semiconductor laser 20.
Assuming that the thickness of the substrate 101 of alumina (thermal conductivity: 33 W/m/K) is 254 xcexcm and the thickness of the conductor films 102 and 103 of gold (thermal conductivity: 315 W/m/K) is 3 xcexcm in the microstrip line shown in FIGS. 18 and 19, thermal conduction between the alumina part and the conductor parts is about 9:1 and a larger quantity of heat is transmitted through the alumina part. While thermal conductivity can be lowered by reducing the thickness of the substrate 101 consisting of alumina, the substrate 101 is readily cracked if the thickness thereof is reduced. Therefore, the thermal conductivity cannot be much reduced in practice.
Wires 90a for the driver IC 30 define another heat inflow path, as shown in FIG. 21. The wires 90a connected to the driver IC 30 for amplifying the electric signal input in the semiconductor laser 20 are formed on an electric circuit mounting unit 90A. When the electric signal is input in the connector 40, the wires 90a for the driver IC 30 must be coupled to leads 90C arranged oppositely to the connector 40. Therefore, the wires 90a are electrically connected to the leads 90C through wires 90d located on the mounting unit 50 and conductor patterns 90b located on a lead mounting substrate 90B.
In the conventional optical module, the wires 90a are coupled to the leads 90C located oppositely to the driver IC 30 through the mounting unit 50 mounting the semiconductor laser 20, and hence heat flows into the mounting unit 50 through the wires 90a or other wires. When the ambient temperature for the optical module changes, therefore, the temperature of the semiconductor laser 20 disadvantageously changes although the temperature of the optical module is so controlled that the temperature detected by the thermistor 70 is regularly constant, similarly to the aforementioned case where heat flows into the optical module through the feeder line 110.
Temperature dependency of the oscillation wavelength of the semiconductor laser 20 is about 10 GHz/xc2x0C., and hence the temperature thereof must be controlled with precision of not more than about 0.1xc2x0 C. within the category temperature range. Therefore, wavelength change of the semiconductor laser 20 caused by heat flowing into the mounting unit 50 through the feeder line 110 or the wires 90a comes into question.
Further, a conventional optical communication system performs no multiplexing in the wavelength region, and hence thermal design is simply based on whether or not the mounting unit 50 can be heated/cooled to a prescribed temperature in the category temperature range. Therefore, the aforementioned problem has been first clarified when the high-density wavelength division multiplex transmission system performing high-density multiplexing in the wavelength region has been watched with interest.
An object of the present invention is to provide an optical module capable of suppressing wavelength change of a semiconductor laser caused by inflow of heat in a high-density wavelength division multiplex optical transmission system multiplexing a plurality of signals in an optical wavelength region in high density and transmitting the same.
The optical module according to the present invention comprises a package, an optical device, a mounting unit and a feeder line. The package has an electric signal input/output unit. The optical device is arranged in the package. The mounting unit mounts the optical device. The feeder line is employed for electrical connection between the optical device and the electric signal input/output unit, and includes a dielectric substrate having thermal conductivity smaller than the thermal conductivity of aluminum oxide and a conductor film formed on the dielectric substrate.
In the optical module according to the present invention, the thermal conductivity of the dielectric substrate employed in the feeder line is smaller than the thermal conductivity of aluminum oxide. Thus, heat can be inhibited from flowing into the optical device through the feeder line as compared with a conventional feeder line employing a dielectric substrate of aluminum oxide. Therefore, a semiconductor laser can be inhibited from wavelength change caused by heat flowing into the same also in a high-density wavelength division multiplex transmission system multiplexing a plurality of signals in an optical wavelength region in high density and transmitting the same.
In the aforementioned optical module, the thermal conductivity of the dielectric substrate is preferably not more than 3 W/m/K.
Thus, the thermal conductivity can be remarkably reduced as compared with the thermal conductivity (33 W/m/K) of alumina employed for a conventional dielectric substrate. Therefore, heat can be further inhibited from flowing into the optical device through the feeder line.
The aforementioned optical module preferably further comprises a thermoelectric cooling element for controlling the temperature of the mounting unit and a temperature detection element mounted on the mounting unit.
According to the present invention, heat can be inhibited from flowing into the optical device through the feeder line as hereinabove described, whereby the temperature of the optical device can be kept constant by controlling the temperature of the mounting unit with the thermoelectric cooling element in response to the temperature detected by the temperature detection element.
In the aforementioned optical module, the material of the dielectric substrate is preferably glass ceramic containing silicon dioxide.
Thus, thermal conductivity lower than that of aluminum oxide, particularly thermal conductivity of not more than 3 W/m/K can be implemented.
In the aforementioned optical module, the line width of the conductor film in a portion not in contact with the package and the mounting unit is preferably smaller than the width of the dielectric substrate.
Thus, heat can be inhibited also from flowing into the optical device through the conductor film.
In the aforementioned optical module, the conductor film for signal transmission is preferably formed only on one of a pair of opposite surfaces of the dielectric substrate.
Thus, heat can be inhibited from flowing into the optical device through the conductor film.
In the aforementioned optical module, the feeder line is preferably either a coplanar line or a slot line.
Thus, the feeder line can be properly prepared from any line in response to the application thereof.
In the aforementioned optical module, the conductor film preferably has a first terminal portion for electrical connection with the optical device, a second terminal portion for electrical connection with the electric signal input/output unit and a connection portion connecting the first and second terminal portions with each other. The connection portion consists of a material having smaller thermal conductivity than that of gold, and the first and second terminal portions consist of gold.
When the connection portion is prepared from a material having smaller thermal conductivity than gold, heat can be inhibited from flowing into the optical device through the conductor film while soldering or bonding of wires or ribbons is enabled on the first and second terminal portions.
In the aforementioned optical module, the conductor film preferably has a first terminal portion for electrical connection with the optical device, a second terminal portion for electrical connection with the electric signal input/output unit and a connection portion connecting the first and second terminal portions with each other. Each of the first terminal portion, the second terminal portion and the connection portion have first layer consisting of a material including at least one element selected from a group consisting of nickel, platinum, palladium, tungsten, molybdenum and copper. Each of the first and second terminal portions have second layer consisting of gold formed on the first layers.
Thus, heat can be inhibited from flowing into the optical device through the conductor film, and soldering or bonding of wires or ribbons is enabled on the first and second terminal portions.
The aforementioned optical module preferably further comprises an electric circuit for amplifying an electric signal input in the optical device, an electric circuit mounting unit mounting the electric circuit and a wire formed on the electric circuit mounting unit and electrically connected to the electric circuit. The wire is electrically connected to a lead located outside the package without through the mounting unit.
Thus, heat can be prevented from flowing into the optical device through the wire electrically connected to the electric circuit.
The aforementioned optical module preferably further comprises a lead mounting substrate mounted on the package and having a conductor pattern for electrical connection to the lead. The lead mounting substrate has an extension part extending toward the electric circuit mounting unit. The conductor pattern extends onto the extension part to be electrically connected to the wire.
Thus, heat can be prevented from flowing into the optical device through the wire electrically connected to the electric circuit, while the conductor pattern and the wire can be electrically connected with each other without requiring a specific member.